Bardsey's Wildlife 2019

BREEDING BIRDS

BREEDING CATEGORIES:

STATUS DEFINITION
Scarce 1-10 pairs breeding annually
Uncommon 11-50 pairs breeding annually
Fairly common 51-250 pairs breeding annually
Common 251-1000 pairs breeding annually
Abundant 1001+ pairs breeding annually

SYSTEMATIC LIST OF BREEDING SPECIES

C A N A D A G O O S E Branta canadensis Scarce breeder on Ynysoedd Gwylan

This year two pairs of Canada Geese were present on Bardsey in spring but did not show any signs of
nesting and neither produced any offspring.

A nest was found on Ynys Gwylan Fawr on 6 June with one half-grown dead chick; additionally, four
young were counted with adults just off the coast of Ynys Gwylan Fawr.

S H E L D U C K Tadorna tadorna Scarce

Six pairs were present this season. The first brood was nine ducklings seen on Solfach on 12 May. This
brood reduced to five ducklings by the next day, and the rest were predated within a week. There were
23 chicks counted throughout the season, presumably the ducklings were predated by gulls, crows and
seals so rapidly that many of the brood sizes were ones or twos before being recorded. The longest
surviving duckling was on Solfach on 13 July and lasted until 30 July before disappearing. Remarkably,
this chick’s growth was so stunted that even after all this time it barely looked more than a few days old.
A burrow with a very large entrance was on the bank at Solfach and footprints around the entrance
suggested it was occupied by Shelducks.

№ pairs with young 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of fledglings 3 6 5 5 3 2 2 4 2 6
8 0 0 13 2 0 0 0 0 0

M A L L A R D Anas platyrhynchos Scarce, population stable

A total of 15 broods was located, one less than 2018. Broods were found from April through to July. Seven
on Pwll Cain, one on East Side, one on Pwll Gareth, two on Nant Pond, two on Solfach, one on Tŷ Pellaf
Pond and one on Henllwyn. There was a minimum of 115 ducklings throughout the season, giving an
average brood size of 7.67. Duckling survival rates were very low as usual, often predated by Ravens,
Carrion Crows and gulls. Five fledge-sized juveniles were present in the Wetlands on 26 July and a female
with five large chicks was on Henllwyn on 9 August.

C U C K O O Cuculus canorus Occasionally breeds as a brood parasite

A male bird was calling at Nant for three consecutive days from 29 April. On 9 May a male and female
were present. On 9 July a juvenile was recorded at Nant and another was seen in the Withies on 28 July
and also at Tŷ Pellaf on 29 July; whether or not these are different birds is unknown.

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W O O D P I G E O N Columba palumbus Scarce, population stable

A total of 13 territories was located this year, one more than last year and 17.1% above the ten-year
mean (11.10±s.d.2.08) and 102.81% greater than the long-term average from 1953-2019 (6.41 ±s.d.4.68).
In total, just seven juveniles were counted throughout the late summer.

Number of pairs 20
15

10

5

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953
Woodpigeon Year
Mean 6.41 ±s.d.4.68

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of pairs 10 9 9 9 10 11 13 15 12 13

M O O R H E N Gallinula chloropus Scarce/uncommon, population fluctuating

Seven clear pairs were recorded this year, a decrease of six compared to the previous year and 23%
below the ten-year mean (9.20±s.d.3.61). A total of 12 broods containing 25 young was recorded, giving
an average brood size of 2.08. Seven reached fledgling size, giving a productivity of 1.00, which is 37.1%
below the ten-year mean (1.59±s.e.0.23).

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Productivity 2.62 1.63 1.25 2.00 3.00 0.75 1.42 1.08 1.15 1.00

O Y S T E R C A T C H E R Haematopus ostralegus Fairly common, population stable

72 pairs nested this year, largely around the coastline on the large rocks as well as the shingle. 25 pairs
nested in the fields and around gorse bushes. This total is 17.4% below the ten-year mean (87.20±s.d.8.44),
but 11.59% greater than the long-term mean from 1953-2019 (64.52 ±s.d.19.96). Of 39 nests that were
monitored, 93 eggs were counted, giving an average clutch size of 2.38. This is 7.2% below the ten-year
mean (2.55±s.d.0.21). A minimum of 29 juveniles were known to have fledged this year, giving a
productivity figure of 0.40 young per pair. This is 12.5% below the ten-year mean (0.46±s.e.0.10).

Number of pairs 120
100
1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
80
60 Oystercatcher Year
40 Mean 64.52 ±s.d.19.96
20

0
1953

№ of pairs 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Mean clutch 94 85 85 87 101 98 82 84 84 72
2.76 2.71 2.75 2.69 2.63 2.36 2.14 2.46 2.65 2.38
Productivity 0.36 0.27 1.15 0.78 0.38 0.07 0.46 0.31 0.39 0.40

144 REP. BARDSEY BIRD FLD OBS

BREEDING BIRDS

K I T T I W A K E Rissa tridactyla Fairly common, population declining

This year saw 121 AON, 6% above the 2010-2019 mean (114.10±s.d.34.72) but 18.45% below the long-term
1953-2019 average (148.38 ±s.d.103.62). On 28 July 137 young were counted, including juveniles and
chicks. There was a minimum of 31 fledglings from 36 AON at the study plot, which gives a productivity
of 0.86, some 52% above the ten-year mean (0.57±s.e.0.09). A minimum of 101 fledged along the full
length of the East Side, giving a productivity for the whole island of 0.84.

Kittiwake and chick © Lewis Hooper
400

Number of pairs 300

200

100

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953
Kittiwake Year

Mean 148.38 ±s.d.103.62

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019

№ of pairs 128 188 76 62 104 125 122 125 90 121

№ of young - 16* 23* 27* 20* 52* 27* 9* 36* 101*

Productivity - 0.52 0.52 0.71 0.21 0.84 0.41 0.11 0.92 0.86

*Only partial census carried out; more birds may have fledged. Productivity calculated at study plots from a smaller number of

AON.

G R E A T B L A C K - B A C K E D G U L L Larus marinus Scarce

Two pairs nested on Bardsey this year, 52.4% below the ten-year mean (4.50±s.d.1.90). A minimum of two
young fledged giving a productivity of 1.00 which is 42.2% above the 2010-2019 mean (0.70±s.e.0.17).

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№ of pairs 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Productivity 2 4 6 8 5 6 4 5 3 2

0.00 1.00 0.50 0.63 0.20 1.83 1.00 0.20 0.67 1.00

As usual Ynys Gwylan Fawr was far more productive than Bardsey, with 40 nests and 32 juveniles. Counts
on Ynys Gwylan Bach produced ten nests with nine juveniles. The sum of these two sites makes this total
of 50 pairs, a 12.5% decrease on the 2010-2019 mean (57.13±s.d.21.28). The combined productivity of the
two islands is 0.82, which is 3.8% above the 2010-2019 mean (0.79±s.e.0.06).

Great Black-backed Gull © Steven Stansfield

№ of pairs 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Productivity 35 44 - - 79 92 72 35 50 50
1.17 0.93 - - 0.75 0.64 0.79 0.68 0.54 0.82

H E R R I N G G U L L Larus argentatus Common, population recovering

A total of 345 AON was counted, 9.4% below the ten-year mean (377.40±s.d.32.62) and 4.81% greater
than the long-term average (329.18±s.d.144.83), but the lowest since 2016’s 331 pairs. A minimum of 245
juveniles was recorded, giving a productivity of 0.71 - 7.8% above the 2010-2019 mean (0.66±s.e.0.03).

Number of pairs 700
600
500 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
400
300 Herring Gull Year
200 Mean 329.18 ±s.d.144.83
100

0
1953

146 REP. BARDSEY BIRD FLD OBS

BREEDING BIRDS

№ of pairs 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Productivity 389 398 395 323 399 400 331 377 417 345
0.68 0.62 0.46 0.64 0.69 0.54 0.84 0.64 0.77 0.71

A total of 89 pairs nested on Ynysoedd Gwylan, 18.6% below the 2010-2019 mean (109.38±s.d.39.84).
Using a sample of 41 pairs at the north end of Ynys Gwylan Fawr, a productivity of 0.76 can be calculated,
1.5% below the 2010-2019 mean (0.77±s.e.0.07).

№ of pairs 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Productivity 115 135 - - 61 52 113 145 165 89
0.78 0.45 - - 1.15 0.54 0.88 0.84 0.76
-

L E S S E R B L A C K - B A C K E D G U L L Larus fuscus Fairly common, population
recovering

This year saw 164 AON, a 7.3% decrease on 2018 and 19.0% below the 2010-2019 mean (202.40±s.d.48.16)
and 13.25% lower that the long-term average (189.05±s.d.193.43). A minimum of 63 young fledged
(compared to 99 last year); the resulting productivity of 0.38 is 15.2% above the 2010-2019 mean
(0.33±s.e.0.06).

№ of pairs 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Productivity 251 209 188 167 315 212 177 164 177 164
0.11 0.13 0.37 0.44 0.59 0.04 0.38 0.56 0.38
-

Number of pairs 700
600
500 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
400
300 Year
200 Lesser Black-backed Gull 189.05 ±s.d.193.43
100

0
1953

G U I L L E M O T Uria aalge Common, becoming abundant, Bardsey population increasing

A total of 1413 AOL is 16.1% above the 2010-2019 mean (1217.56±s.d.201.33), This is the second largest
count ever and 217.84% greater than the long-term average from 1953-2019 (444.56 ±s.d.421.37). At least
63 fledge-able sized chicks were counted in a sample plot of 110 AOL at Bae Felen, giving a productivity
figure of 0.57 - 78.7% above the 2010-2019 mean (0.32±s.e.0.03).

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019

№ of AOL 987 981 1234 1071 1141… 1240 1346 1574 1112 1413

№ of young - 48* 18* 22* 51* 18* 50* 70* 94* 63*

Productivity 0.30 0.30 0.22 0.28 0.26 0.38 0.26 0.40 0.22 0.57

*Only partial census carried out; more chicks fledged from other parts of the island. Productivity calculated at study plots from a
smaller number of AOL. … Includes estimate from uncounted colonies

On 6 June, 27 AOL were counted on Ynys Gwylan Fawr and 54 AOL were counted on the south west
side of Ynys Gwylan Bach. The combined total of 81 is 28.3% above the 2010-2019 mean (63.13±23.54).
However, no meaningful sample plots were studied to calculate productivity.

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2000

Number of AOL 1500

1000

500

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953
Guillemot aol Year
Mean 444.56 ±s.d.421.37

№ of AOL 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of young 104 77 - - 32 44 51 49 67 81
Productivity - - - 1 - - 23 - -
- - - - 0.03 - - - - -
0.49

R A Z O R B I L L Alca torda Common to abundant, population recovering

There were 1917 AOL, based on flush counts, with x0.67 conversion factor applied (as per Walsh et al.,
1995), this is 2.8% below the 2017 population count. A minimum of 243 young and 68 eggs were counted
along the East Side, which combined is a 2.3% increase on the ten-year mean (303.20±s.d.40.65), and
14.67% greater than the 1953-2019 average (271.22 ±s.d.114.20).

Number of pairs 700
600
500 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
400
300 Razorbill Year
200
100 Mean 271.22 ±s.d.114.20

0
1953

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Eggs/young 295 400 278 287 300 317 321 246 277 311

Once again, access to Ynys Gwylan Bach was not possible and this meant that counts on Ynys Gwylan
Fawr yielded 13 AOL, with 7 chicks and 2 eggs recorded.

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Eggs/young 15 17 - - 18 - 24 42 16 9

P U F F I N Fratercula arctica Uncommon, increasing on Bardsey, stable on Gwylan

This year there were a minimum of 143 AOBs, of which 101 were on the grassy cliffs above Ogof Morlas,
with 42 between Bae Felen and Seal Cave. There were also birds seen regularly further south along the
East Side and they were presumed to have nested near Pen Cristin and Ogof Barcut, but no active
burrows were found. This is 115.4% above the 2010-2019 mean (66.40±s.d.47.07); a dramatic increase for
the species in the short time since they colonised Bardsey.

148 Razorbills © Lewis Hooper

REP. BARDSEY BIRD FLD OBS

BREEDING BIRDS

6 3 : 16104 0 - 1 6 3s/young149
120

PRETTYMAN AND STANSFIELD

Estimated 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
population 23 16 16 29 55 75 75 100 132 143

Very dense vegetation in some areas of Ynys Gwylan Fawr meant the figure of 619 AOB was
undercounted, especially the steeper slopes to the south west of the island. This was an estimated 7.7%
decrease on the 2009-2019 mean (686.50±s.d.201.64).

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
839* 871 - 682 663 819 619*
AOB 240* 759* -

* Only partial census completed for safety reasons

S T O R M P E T R E L Hydrobates pelagicus Fairly common

Adults were heard calling in the usual spots around Briw Gerrig, Seal Cave and Bae Felen in June and
July. One bird was seen on a nest in an artificial nest box, though it appeared to have failed after the
chick had hatched.

F U L M A R Fulmarus glacialis Uncommon, population fluctuating

Trips to the East Side of Bardsey throughout the Fulmar nesting season yielded counts adding to 14 AOS,
equal to the 2018 total, but 23.1% below the ten-year average (18.30±s.d.5.66) and 29.40% lower than
the long-term mean 1953-2019 (19.83 ±s.d.15.46). Unfortunately, weather conditions later in the season
when the young birds will have fledged meant that a count of fledglings was not recorded and
consequently a productivity figure was not calculated.

Apparently Occupied Sites 60
50
40 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
30
20 Fulmar Year
10 Mean 19.83 ±s.d.15.46

0
1953

№ of AOS 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Productivity 18 22 21 11 17 31 15 20 14 14
- 0.40 - - - - - - - -

M A N X S H E A R W A T E R Puffinus puffinus Abundant, population stable

From 122 monitored study burrows, 94 young successfully fledged. The productivity figure of 0.77 is 12.9%
above the ten-year mean (0.68±s.e.0.02), similar to other large colonies. A full report on the breeding
season for Manx Shearwaters can be found on page 164.

Productivity 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Sample size 0.76 0.73 0.72 0.61 0.70 0.64 0.64 0.65 0.60 0.77
90 122 148 132 101 119 125 127 125 122

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BREEDING BIRDS

S H A G Phalacrocorax aristotelis Uncommon, population increasing again

Shag nests were counted from the land across several visits to the East Side of Bardsey. A total of 45 nests
was counted. This is equal to 2018 and 2017, the joint highest total since 2006 and 22.6% above the 2010-
2019 mean (37.60±s.d.6.65) and 46.39% greater than the long-term mean from 1953-2019 (30.74
±s.d.10.99). In total, 73 fledged young were counted, giving a productivity of 1.62, which is 16.7% below
the 2010-2019 mean (1.94±s.e.0.13).

Number of pairs 80
70
60 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
50
40 Shag
30
20
10

0
1953

Year

Mean 30.74 ±s.d.10.99

№ of pairs 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Productivity 25 39 34 31 34 39 39 45 45 45
2.60 2.08 2.59 2.00 1.65 1.28 1.71 1.82 2.09 1.62

In contrast to the steadily increasing population on Bardsey, only 52 AON were found on Ynys Gwylan
Fawr, this is 14 fewer nests than in 2018, a decline of 21.2%. This year’s total of AON is, however, a 5.2%
increase on the 2009-2018 mean (49.44±s.d.14.56). In total 106 young were found on Ynys Gwylan Fawr
alone, giving a productivity figure of 2.04, which is 18% above the ten-year mean (1.73±s.e.0.12). Due to
tides and weather conditions a trip to Ynys Gwylan Bach was not possible.

№ of pairs 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Productivity 75 49 - 24 46 48 41 44 66 52
2.05 2.12 - - 1.98 1.59 1.66 1.15 1.23 2.04

C O R M O R A N T Phalacrocorax carbo Uncommonon Ynysoedd Gwylan

Two visits to Ynys Gwylan Fawr were made on 6 and 19 June. However, Cormorants, as in recent years,
have only nested on Ynys Gwylan Bach, and weather and tide conditions did not allow access to this
island, so photographs were taken of the colony and counts made retrospectively. A minimum of 20
AON was recorded, 81.8% above the 2010-2019 mean (11.00±s.d.4.27). 25 juveniles were counted, giving
a productivity of 1.25, which is 11.8% lower than the 2010-2019 mean (1.42±s.e.0.17).

Year 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Y G Fawr pairs 41 34 - 31 0 0 0 0 0 0
Y G Bach pairs 10 12 - 5 9 9 8 12 14 20
51 46 - 36 9 9 8 12 14 20
Total pairs 0.92 2.15 - -
Productivity 1.56 1.44 2.13 1.25 0.64 1.25

P E R E G R I N E Falco peregrinus Scarce (Schedule 1), population stable

One nest was located on 10 May, with a female sitting in one of the known nest sites. Due to the location
of the nest the contents were not recorded. On 1 July two juveniles were seen flying over the Mountain
opposite the nest site; this gives a productivity of at least 2.00.

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№ of pairs 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Productivity 2 2 2 2 2 2 2 1 1 1
0 0
1.00 2.50 2.00 2.00 2.00 1.00 1.00 2.00

L I T T L E O W L Athene noctua Scarce, population stable

Four males were holding territories in April, but this dropped to two by the start of June, when birds were
heard calling throughout the season on a daily basis, both at Nant Valley East and behind Carreg Bach.
This is one fewer territory than 2018 and 47.4% below the ten-year mean (3.80±s.d.1.69). Only one juvenile
was known to have fledged and was present in the gorse in Nant Valley for two weeks in July, giving a
productivity of 0.50. 2016 was the last year in which any young had been recorded, but they easily go
undetected.

Little Owl © Lewis Hooper

10

Number of pairs 8

6

4

2

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953
Little Owl Year
Mean 4.35 ±s.d.1.73

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of pairs 4 3 3 2 3 5 7 6 3 2

M A G P I E Pica pica Scarce, population fluctuates, but fairly stable

Ten pairs nested this year, predominantly in gorse patches around the island, both on the Mountainside
and at the North End. However, there were also nests in the Plantation and Carreg and Plas Withies. A
total of 28 juveniles was counted, giving a productivity of 2.80 young per pair, 74.5% above the ten-year

152 REP. BARDSEY BIRD FLD OBS

BREEDING BIRDS

mean (1.61±s.e.0.23). This is the third consecutive year that the population has fallen and is now 10.7%
lower than the ten-year mean (11.20±s.d.2.44), but it is still greater than the long-term average 1953-2019
(7.34 ±s.d.3.33).

Number of pairs 16
14
12 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
10
Magpie Year
8 Mean 7.34 ±s.d.3.33
6
4
2
0

1953

№ of pairs 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Productivity 9 8 10 11 9 14 15 14 12 10
1.80 1.55 0.93 0.87 1.57 2.83 2.80
1.67 1.13 0.90

C H O U G H Pyrrhocorax pyrrhocorax Scarce (Schedule 1), population stable

Seven pairs nested or attempted to nest this year, one less than last year, 13.6% below the ten-year mean
(8.10±s.d.0.99), and 51.52% greater than the long-term average (4.62 ±s.d.2.20). Two pairs fledged three
young, one pair fledged four young and one pair fledged one young. The total of 11 juveniles almost
matches the 2010-2019 mean (15.40±s.d.4.81), and the resulting productivity figure of 1.57 is 16.6% below
the 2010-2019 mean (1.88±s.e.0.16).

10

Number of pairs 8

6

4

2

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953
Chough Year
Mean 4.62 ±s.d.2.20

№ of pairs 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of young 6 8 9 9 8 9 8 9 8 7
Productivity 8 14 15 13 24 18 14 22 15 11

1.33 1.75 1.67 1.44 3.00 2.00 1.75 2.44 1.88 1.57

C A R R I O N C R O W Corvus corone Scarce, population stable

Seven pairs nested this year, two less than last year and the lowest since 2014, before the population
boosted to 13 pairs from 2015 through to 2017. A minimum of 14 young fledged this year, giving a
productivity of 2.00.

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of pairs 9 7 9 10 7 13 13 13 9 7

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Number of pairs 14
12
10 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018

8 Carrion Crow Year
6 Mean 6.83 ±s.d.2.45
4
2
0

1953

Carrion Crow chicks © Lewis Hooper

R A V E N Corvus corax Scarce

One pair nested at Ogof Barcut this year with a brood of three, although only two chicks were known to
have fledged on 23 May. There were seemingly two pairs on the island during the months of April and
May, the second pair were consistently seen at North End, but no nesting attempt was ever recorded.

5

Number of pairs 4

3

2

1

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953
Raven Year
Mean 1.15 ±s.d.0.80

154 REP. BARDSEY BIRD FLD OBS

BREEDING BIRDS

S W A L L O W Hirundo rustica Uncommon, population decreasing

Ten pairs nested this year, one more than the previous year, but still 35.9% below the ten-year mean
(15.60±s.d.4.33). However, it is 19.62% greater than the long-term mean (8.36±s.d.4.81). The first nest was
found at Tŷ Pellaf on 11 May, shortly followed by two more at Tŷ Pellaf, one in the Boathouse, two in Lloft
Plas shed, two at Cristin and two at Nant. Most pairs managed to fledge two broods, a minimum total of
69 young was recorded, 36 from the first brood and 33 from the second brood, to give a productivity
figure of 6.90 young per pair, this is 35% above the ten-year mean (5.11±s.e.0.50).

Number of pairs 25
20
15 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
10
Swallow Year
5
0 Mean 8.36 ±s.d.4.81

1953

№ of pairs 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of young 12 21 19 18 17 21 14 15 9 10
Productivity 42 106 100 62 105 77 79 54 71 69
3.50 5.05 5.26 3.44 6.18 3.67 5.64 3.60 6.90
7.89

H O U S E M A R T I N Delichon urbicum Scarce occasional breeder

Five pairs bred at the Lighthouse this year, three more than last year and 38.9% above the ten-year mean
(3.60±s.d.2.67). A minimum of 20 young fledged, giving a productivity of 4.00. This year young were
fledging as late as 8 September.

20

Number of pairs 15

10

5

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953
House Martin Year

Mean 1.23 ±s.d.3.14

C H I F F C H A F F Phylloscopus collybita Scarce

Three pairs nested this year; singing birds were heard on a daily basis throughout the months of May and
June, but until mid-June when food-carrying adults were seen at Cristin and Nant, breeding could not
be confirmed. A total of nine juveniles was recorded, two at Cristin on 28 June, three being fed by an
adult in the Plantation on 1 July, and four being fed by an adult on 10 July. The three pairs this year was
42.9% greater than the ten-year mean 2.10±s.d.1.37, and 240% greater than the long-term average (0.88
±s.d.1.24).

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5

Number of pairs 4

3

2

1

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953
Chiffchaff Year
Mean 0.88 ±s.d.1.24

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of pairs 4 1 4 2 3 1 0 2 1 3

W I L L O W W A R B L E R Phylloscopus trochilus Common migrant, occasional breeder

Two very persistent birds were singing at the Plantation and Cristin throughout the month of May. These
were the only signs of possible breeding until a young juvenile was caught at the Plantation on 15 July.

5

Number of pairs 4

3

2

1

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953
Willow Warbler Year
Mean 0.33 ±s.d.0.76

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of pairs 0 4 0 0 2 0 0 0 0 1

S E D G E W A R B L E R Acrocephalus schoenobaenus Scarce

Seven pairs held territories this year, an increase of two on last year, and 2.9% above the ten-year mean
(6.80±s.d.2.66). This year the pairs were found in Carreg and Tŷ Pellaf reed beds, in the wetlands and also
on the Mountain, where birds were recorded food carrying and singing on multiple occasions throughout
May and June. A minimum total of nine juveniles was recorded, in Carreg reed bed, Tŷ Pellaf reed bed
and Tŷ Pellaf Withy.

Number of pairs 12
10
1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
8
6 Sedge Warbler Year
4 Mean 1.44 ±s.d.2.57
2
0

1953

156 REP. BARDSEY BIRD FLD OBS

BREEDING BIRDS

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of pairs 3 8 8 9 3 5 10 10 5 7

W H I T E T H R O A T Sylvia communis Scarce

Six pairs held territories this year and of those six, at least four fledged offspring. The successful pairs nested
on the Mountain behind Cristin, Cristin, around the Lime Kiln and Nant Withy. The earliest signs of breeding
came on 15 May with a food-carrying male north of Nant Withy. A total of ten juveniles was counted,
the first of which was seen on 1 June.

Number of pairs 16
14
12 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
10
Whitethroat Year
8 Mean 3.11 ±s.d.3.57
6
4
2
0

1953

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of pairs 1 5 7 6 7 1 2 2 1 6

G O L D C R E S T Regulus regulus Occasional breeder, 1999, 2007 and 2014-19

On 15 July two adult males and two females with wrinkled brood patches were trapped in the Sitka
plantation at Nant along with four juveniles; a further four juveniles were also present. This is the sixth
consecutive year this species has nested at Nant in the Sitkas and shows the value of the mature
plantation.

3

Number of pairs 2

1

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953
Goldcrest Year
Mean 0.16 ±s.d.0.44

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of pairs 0 0 0 0 1 2 1 1 1 2

W R E N Troglodytes troglodytes Fairly common, population fluctuates

There were 108 territories of singing males recorded during the Breeding Bird Census (BBS) dawn survey,
82 of which were multiple registrations and 26 were single registrations. This total is down 19.4% from last
year’s total of 134 territories and 23.2% below the ten-year mean (140.60±s.d.49.16). The population has
generally been decreasing by a rate of about 19 pairs per year since 2015. Furthermore, the exceedingly

63: 140-163 157

PRETTYMAN AND STANSFIELD

cold weather in early 2018 almost halved the population from 2017 and it is possible that this event is still
taking its toll on the breeding Wrens.

Number of pairs 300
250
200 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
150
100 Wren

50
0
1953

Year

Mean 52.38 ±s.d.49.91

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of territories 107 81 121 101 159 186 163 246 134 108

B L A C K B I R D Turdus merula Scarce/uncommon, population increasing

This year a total of 14 pairs was recorded, 8.5% above the ten-year mean (12.90±s.d.4.20), but five fewer
pairs than last year. For the fifth consecutive year the number of pairs has been higher than the long-
term average (13.09±s.d.12.94). A minimum of 45 young fledged, giving a productivity figure of 3.21,
36.7% above the ten-year mean (2.35±s.e.0.28). The majority of birds appeared to nest in the island’s
gardens, although there were also many nesting birds in the wetlands. The first juvenile was caught at
Cristin on 17 May.

50

Number of pairs 40

30

20

10

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953
Blackbird Year
Mean 13.09 ±s.d.12.94

№ of pairs 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Productivity 5 7 12 16 12 14 14 16 19 14
2.83 1.56 1.83 1.71 2.14 1.05 2.05 3.21
3.40 3.71

S T O N E C H A T Saxicola torquatus Scarce, population recovering / stable

Stonechats had a good year, with ten breeding pairs, six more pairs than last year. This was 56.3% above
the ten-year mean (6.40±s.d.3.13) and 92% higher than the long-term average 1953-2019 (5.20 ±s.d.3.96).
One pair was seen having their nest predated by Magpies, but despite this, 52 juveniles were recorded,
giving a productivity figure of 5.20, 18.1% below the ten-year mean (6.35±s.e.0.64).

№ of pairs 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of young 1 3 7 5 7 10 10 7 4 10
Productivity 10 29 44 28 43 49 52 26 27 52
4.90 5.2 5.20
10.0 9.67 6.29 5.60 6.14 3.71 6.75

158 REP. BARDSEY BIRD FLD OBS

BREEDING BIRDS

W H E A T E A R Oenanthe oenanthe Scarce/uncommon, population recovering

This year a total of 23 pairs was recorded, five more than last year, but paling in comparison with the
2017 record total of 38 pairs. However, despite the high numbers of over 27 pairs from 2014-2017, this
year’s total is still 4.1% above the ten-year mean (22.10±s.d.11.28) and 56.78% greater than the long-term
average from 1953-2019 (14.67 ±s.d.9.59). A minimum of 36 young fledged this year, giving a productivity
figure of 1.57 young per pair; this is 29.8% below the ten-year mean (2.24±s.e.0.28). Of the four known
nests, all had two broods. The first breeding behaviour was recorded on 25 April and the first juvenile bird
was seen on 1 June.

40

Number of pairs 30

20

10

0 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
1953
Wheatear Year
Mean 14.67 ±s.d.9.59

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
18 23
№ of pairs 6 11 11 17 34 35 28 38 46 36
2.56 1.57
№ of young 16 27* 28 55 65 54 31 51

Productivity 2.67 3.86* 2.55 3.24 1.91 1.54 1.11 1.34

*Number of young/productivity was from a sample of monitored nests and not the entire population.

D U N N O C K Prunella modularis Uncommon, population fluctuating

Ten males held territory this year, which is one more than last year, but 58.3% below the ten-year mean
(24.20±s.d.14.51). As displayed in the graph below, the ten-year mean is high due to a large spike in the
population from 2010 to 2015. The total this year is 28.42% below the long-term mean (13.97±s.d.8.28). A
minimum of 17 juveniles were recorded this year, the majority coming from the Mountainside, including
at least three broods at Cristin.

Number of pairs 60
50
40 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
30
20 Dunnock Year
10 Mean 13.97 ±s.d.8.28

0
1953

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of males 29 34 49 45 20 20 11 15 9 10

P I E D W A G T A I L Motacilla alba Scarce, population stable

Six pairs nested this year, which is equal to last year, but low compared to 14 pairs that were present in
2015. This total is 35.48% below the ten-year mean (9.30±2.45), but 35.70% greater than the long-term

63: 140-163 159

PRETTYMAN AND STANSFIELD

average 1953-2019 (4.42 ±s.d.3.45). A minimum of 21 juveniles were recorded, giving a productivity figure
of 3.50, which is 2.1% below the ten-year mean (3.57±s.e.0.21).

Number of pairs 16
14
12 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
10
Pied Wagtail Year
8 Mean 4.42 ±s.d.3.45
6
4
2
0

1953

№ of pairs 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of young 7 11 10 10 10 14 10 9 6 6
Productivity 24 42 31 48 46 40 29 30 17 21
3.82 3.10 4.80 4.60 2.86 2.90
3.43 3.33 3.40 3.50

M E A D O W P I P I T Anthus pratensis Fairly common, population decreasing

The population dropped severely again this year, and the absence of singing males was noticeable.
Only 46 pairs apparently bred in 42 multiple registration and four single registration territories from the four
BBS visits. This total is 42.5% below the ten-year mean (80.00±s.d.23.00), and 12.94% lower than the long-
term average (52.84 ±s.d.26.62). The first fledged birds were recorded on 13 May in the Lowlands, still
being fed by parents, with 4 August being the last date where young birds were still seen. Populations
have disappeared mainly from the fields rather than the Mountain. Over the past two seasons farming
practices have changed dramatically, in that there are no longer any hay meadows and long sward
during the summer months. When the hay making regime was announced in 2016 it was predicted this
would have a negative effect on the Meadow Pipit population, but at the same time having a positive
effect on other species.

Number of pairs 140
120
100 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018

80 Meadow Pipit Year
60 Mean 52.84 ±s.d.26.62
40
20

0
1953

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of pairs 62 59 66 72 97 118 93 106 81 46

R O C K P I P I T Anthus petrosus Uncommon, population increasing

This year’s total of 42 pairs is two more than last year and 5.4% below the ten-year mean (44.40±s.d.7.69),
but 26.13% greater than the long-term average 1953-2019 (33.30 ±s.d.10.41). Omitting the East Side with
its 11 pairs, 43 juveniles were recorded on the coastline, giving a productivity figure of 1.39 young per
pair.

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BREEDING BIRDS

Number of pairs 70
60
50 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
40
30 Rock Pipit Year
20 Mean 33.30 ±s.d.10.41
10

0
1953

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of pairs 41 31 43 47 50 59 40 51 40 42

C H A F F I N C H Fringilla coelebs Scarce, population declining

Three pairs bred this year, two at Nant and one at Cristin. This is one more pair than last year, some 58.9%
below the ten-year mean (7.30±s.d.4.11), but still 81.82% greater than the long-term average (1.65
±s.d.3.18). This species’ population was bolstered by seed feeders being a food source in the winter; this
explains the increase from 2005 – 2015 when they were being fed by over-wintering residents on Bardsey.
A total of ten juveniles was recorded between 9 June and 27 July.

Number of pairs 14
12
10 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018

8 Chaffinch Year
6 Mean 1.65 ±s.d.3.18
4
2
0

1953

2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of pairs 8 9 11 11 12 11 4 2 2 3

L I N N E T Carduelis cannabina Uncommon, population declining

This year 23 pairs were recorded breeding, which is equal to last year and 17.0% below the ten-year
mean (27.70±8.25) and 3.81% lower than the long-term average (23.91 ±s.d.16.06). Nesting took place
across the island including the Wetlands, Lowlands, South End, North End and the Mountainside. The
Mountain and grassy fields on the West Coast were less productive areas. This year 80 juveniles were
recorded, giving a productivity figure of 3.48 young per pair, 21.6% above the ten-year mean
(2.95±s.e.0.45).

№ of pairs 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of young 17 19 27 31 25 41 30 41 23 23
Productivity 53 106 58 148 68 39 38 114 62 80
3.12 5.58 2.15 4.77 2.72 0.95 1.27 2.71 2.70 3.48

63: 140-163 161

PRETTYMAN AND STANSFIELD

Number of pairs 80
70
60 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
50
40 Linnet Year
30 Mean 23.91 ±s.d.16.06
20
10

0
1953

4 SUMMARY

2019 has shown some interesting changes in breeding populations. Some all too familiar outcomes
occurred, of course. As usual, Shelducks failed to fledge any offspring despite the usual annual attempts,
the chicks being swiftly predated by gulls, crows and seals before they could shed their downy plumage.

Some interesting breeding behaviour was recorded from passerines. During late May into June two
apparent Siskin pairs were caught, the females in both pairs sporting rather large, wrinkled brood
patches. These birds were not recorded after mid-June and they were not included in this report. And it
was not just Siskins that were unusual, Goldcrests bred once again, and this year juveniles were actually
caught before their crests had formed, as well as Willow Warblers, which rarely breed here. The number
of pairs for most species have held steady from 2018. However, there have been some rather dramatic
changes in just a single year. Meadow Pipits are easily the most alarming, although they are difficult to
map, it was quite clear early in the season that numbers were low, and so more effort was put into
mapping the pairs. Despite the added effort, just 46 were recorded which is a drop of 42% from the ten-
year mean. Following the removal of cattle in the winter months, there is no longer any need for hay or
silage to be produced to feed the stock. As a result, much of the land that was turned over to hay
meadows for the previous decade is now grazed in the summer, reducing the suitable habitat for
Meadow Pipits. The population did decline quite severely from 2017 into 2018, and this year saw a
continuation of that same trend. It’s possible that should there be a change in the grazing regime in the
future that the population will bounce back just as quickly as it grew when the cattle were introduced in
larger numbers in the early 2000s.

This season’s seabird breeding populations have remained steady. For the third year running 45 Shag
nests were recorded on Bardsey, an increase on the ten-year average. On the Gwylan Islands, the
Cormorant colony is steadily growing, and has reached a total of 20 pairs. The Great Black-backed Gull
population has also remained steady on Ynys Gwylan Fawr, the highest it has been in the last three years.
Hopefully the improved productivity is a sign of continued growth into the future, although this probably
isn’t such a pleasing statistic to the island’s breeding Puffins. After a drop last year, the Kittiwake
population has realigned with the 2015-2017 population count and the productivity had been an
increase on the ten-year mean. This is similar for Guillemots; last year the population took a slight fall, but
bounced back this year, although the increase that took place from 2012-2017 seems to have ground
to a halt. Puffins on Bardsey continue their uninterrupted population growth, jumping to 143 occupied
burrows, up from 132 in 2018.

For the first time since 2016 some breeding success was noted with Little Owls. Although only two pairs
apparently attempted to breed, which is fewer than in almost every previous year, one juvenile was
making itself very obvious in Nant Valley.

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BREEDING BIRDS

5 REFERENCES

Eagle, G. 2012. Bardsey’s breeding birds, 1992-2011. Rep. Bardsey Bird Fld. Obs. 55: 138-143.
Jones, P.H. 1988. The Natural History of Bardsey. National Museum of Wales, Cardiff.
Loxton, R.G. and Jones, P.H. 1995. The breeding birds of Bardsey, Skomer, Skokholm and the Calf of Man.

Part 1: Introduction and species accounts. Rep. Bardsey Bird Fld. Obs. 38: 84-159.
Mavor, R.A. et al. 2008. Seabird numbers and breeding success in Britain and Ireland, 2006.

Peterborough, JNCC UK Nature Conservation, No 31.
Walsh, P.M. et al. 1995. Seabird monitoring handbook for Britain and Ireland. JNCC/RSPB /ITE/Seabird

Group, Peterborough

Dunnock © Steven Stansfield

63: 140-163 163

PRETTYMAN AND STANSFIELD

MANX SHEARWATER

Productivity, ringing and chick growth rates
Samuel Prettyman

Manx Shearwater chick REP. BARDSEY BIRD FLD OBS

© George Dunbar

164

63: 140-163 165

PRETTYMAN

1 INTRODUCTION

Manx Shearwater is a truly pelagic bird, only returning to land to breed on a small number of select
offshore North Atlantic islands. It is currently identified as a conservation priority in the EC Birds Directive -
migratory species. It is Amber listed in Birds of Conservation Concern 3 (2009 update) (further information
on Conservation Designations for UK Taxa) and is Amber listed in Birds of Conservation Concern in Ireland
2008-2013 (2013 update).

Bardsey is considered to be the fourth largest Manx Shearwater colony in the world and is the third largest
of the three major colonies in Wales, following Skomer and Skokholm. As a significant proportion of the
world’s breeding population of Manx Shearwaters is found on Bardsey it is well-studied on the island. The
breeding population is known, its wintering quarters are known through ringing recoveries, and the
average productivity (number of young fledged per pair) since 1998 is known from a sample of up to
150 burrows monitored annually. Because of the global importance of the colony on the island, the Manx
Shearwater is one of the avian ‘features’ of the island as an SSSI and NNR.

Following a complete census undertaken on the island by Bardsey Bird Observatory between 2014 and
2016, a total of 20,675 Apparently Occupied Burrows (AOBs) were counted (Stansfield and Carter 2017),
showing an increase of almost 5000 AOBs (+30.37%) from the previous census in 2008 to 2010. Data from
year one of the current 2019-2022 census shows a further increase of 19.6%, which means that if the
increase is the same across the whole island, the population would now stand at somewhere in the
region of 25,000 pairs.

Data from this project is used by the Observatory and Natural Resources Wales (NRW) to assess the health
of the population. The data also forms part of the Seabird Monitoring Programme (SMP); established in
1986, this is an ongoing annual programme monitoring 26 species of seabird that regularly breed in Britain
and Ireland. It aims to ensure that sample data on breeding numbers and breeding success of seabirds
are collected, both regionally and nationally, to enable their conservation status to be assessed. The
SMP is led and co-ordinated by the Joint Nature Conservation Committee (JNCC) in partnership with
others including NRW. In November 2009 all partners signed a Statement of Intent that established the
way forward for future data-sharing and collaborative working.

2 AIMS

The aims of this study are to continue the productivity monitoring programme and expand the monitoring
area, to increase the proportion of the ringed population, particularly juvenile birds, and to re-trap as
many individuals as possible. The increased ringing of juveniles will provide details of more birds of known
age, therefore the age of first breeding will be known, as well as juvenile survival and dispersal. The
increased number of re-traps should give a clearer indication of the mean life expectancy, highlight any
losses in particular age classes and provide a robust indication of the population changes.

3 METHODS

3.1 PRODUCTIVITY MONITORING

A sample of Manx Shearwater burrows was selected over the previous few summers to be studied as
part of the long-running productivity monitoring survey. The burrows were visited several times throughout
the nesting season, initially during the mean egg-laying period when the Manx Shearwaters had just
begun to incubate. The next visits were made whilst the adults were rearing their young, and the final
visits were made during the fledging period. Shallow and easily accessible burrows were selected for the
survey so that the contents could be accurately monitored. Accessible burrows containing Manx

166 REP. BARDSEY BIRD FLD OBS

SHEARWATER PRODUCTIVITY, RINGING AND CHICK GROWTH

Shearwaters and eggs were located at a number of clustered sites across the island covering a broad
range of habitats and areas.

Each burrow was mapped and marked with an individually numbered wooden stake and a grid
reference taken to aid relocation, both later in the season and the following year. When the chicks were
large enough, they were ringed, and biometrics were taken (wing length and weight). A final visit was
made to calculate how many chicks had successfully fledged.

3.2. RINGING

In the UK, birds are ringed by specially trained
ringers under licence from the British Trust for
Ornithology (BTO) who, on behalf of the UK’s
government, run the British and Irish Ringing
Scheme and issue the rings for the birds.

3.2.1 RINGING ADULT BIRDS

Adult Manx Shearwaters generally come ashore Fig. 1 Sam ringing a Manx Shearwater
after dark during the new moon period, © Steven Stansfield
therefore, they are trapped at night-time for
ringing as they return to their burrows; head
torches are used to dazzle the birds so that they
cannot see the ringer approaching. They are
then picked up by the ringer and, using specialist
pliers, a uniquely numbered metal ring is fitted to
the bird’s leg. Once the ring has been fitted, the
bird’s age, sex (if possible), biometric data, date,
time and location are all noted. These data are
entered onto Bardsey Bird Observatory’s own
database, and also onto the BTO’s database
and submitted as part of the ringer’s licensing
requirements.

3.2.2 RINGING YOUNG BIRDS

When the chicks are approximately one month old, they are taken from the burrow during the daytime
and ringed using the same process as with the adults. Some of the shorter burrows have chambers which
are around 30cm or so beneath the surface allowing easy access by the ringer. However, burrows of
more than arm’s length may be accessed using a piece of bent fencing wire with a rubber coating on
the end to prevent harm to the bird. The chick is then removed by hooking the bird around its leg, and
gently easing it towards the surface. Many of the birds that nest on the island use chambers which are
several metres underground and are therefore, inaccessible. During late August and early September
some of the more developed young are captured at night when they come out of their burrows to
exercise their wings. These birds are distinguished from adults by several plumage characteristics (mainly
the presence or absence of down on the body). Birds with <20% body cover of down are aged as
juveniles that are able to fledge (Euring age code 3, i.e. hatched during the current calendar year). Birds
with a body cover of down of >20% are aged as pullus (Euring age code 1, i.e. a bird still in its nest). The
birds aged as 3 are technically still pulli but a figure of 20% down cover is used to differentiate between
birds that are capable of flight and ones that are not.

63: 164-177 167

PRETTYMAN

Fig. 2 Adult Manx Shearwater exiting burrow © Steven Stansfield

3.2.3 RE-TRAPPING RINGED BIRDS

When a ringer captures a bird, a preliminary check is made to identify whether the bird is already ringed.
If an existing ring is found, the number is written down and double checked, the time, location, age and
sex are noted, and the bird is then released. These data are then entered onto the BTO’s database as
re-traps. The database is sufficiently powerful to be able to match up all the previous captures of any
individual bird, including its original ringing data, and can provide a complete capture history for each
bird. The physical state of the ring at the time of capture is also examined and, depending on the level
of wear, it may then be replaced; after 15 to 20 years rings can become thin and the numbers can
become illegible, so worn rings are carefully removed with circlip pliers. The numbers of both rings are
noted and later matched up on the database; the original ringing data is then applied to the new ring.
This method of re-ringing is necessary with long-lived birds such as shearwaters, especially since the ring
will be submerged in saltwater for the majority of its life.

3.3 GROWTH RATE OF CHICKS

In addition to monitoring the productivity, 36 of the productivity burrows were chosen for a survey looking
at the growth rate of chicks from hatching through to fledging. The selected nests were visited each
week, the young carefully removed from their burrows, and measurements of wing-length and weight
were taken.

4 RESULTS

4.1 PRODUCTIVITY

This season 122 burrows containing an adult bird incubating an egg were monitored. These were marked
with a red-topped, individually numbered wooden post. Shearwaters from these burrows were followed
through to their conclusion. In total 27 failed at the egg stage, one failed at the chick stage and the
remaining 94 pairs successfully reared young, giving a productivity figure of 0.77 chicks per breeding pair;
an increase of 28.33% on last year’s productivity (0.60), and 12.90% higher than the ten-year mean (0.68
±s.e.0.02). This is 4.67% lower than the mean since 1998 (0.74 ±s.e.0.02) when Bardsey Bird Observatory
began monitoring productivity. The highest productivity was 0.88 in 2003 and the lowest was 0.60 in 2018.

168 REP. BARDSEY BIRD FLD OBS

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0.85

Productivity 0.75

0.65

0.55

1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019

Productivity Year
Mean 0.74 ±s.e.0.02

Fig.3. Manx Shearwater productivity 1998-2019

Table 1 Productivity 2010 – 2019

Year 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
Productivity 0.76 0.73 0.72 0.61 0.70 0.64 0.64 0.65 0.60 0.77
Sample size 90 122 148 132 101 119 125 127 125 122

Table 2 Area specific nest and productivity details

Area Incubating Eggs Fledged Eggs hatched Productivity
(per pair) (chicks per
adults hatched/young 7
8 0.54 pair)
South End 13 7 12 0.53 0.54
Cristin 15 8 1 0.87 0.53
15 13 4 0.50 0.80
Pen Cristin 2 1 11 0.80 0.50
West Coast 5 4 12 0.73 0.80
Mountain North 15 11 3 1.00 0.73
North Stream 12 12 6 0.60 1.00
North West Fields 5 3 4 0.86 0.60
7 6 10 1.00 0.86
Barcut 4 4 16 0.91 1.00
Nant Valley East 11 10 94 0.89 0.91
Nant Valley West 18 16 0.78 0.89
122 95 0.77
Tŷ Pellaf
Mountain South

Total

63: 164-177 169

PRETTYMAN

4.2 RINGING NEW BIRDS

During the new moon periods from mid-March through to September, while the shearwaters were
ashore after dark, a total of 1298 full-grown birds were handled by observatory staff. Of these, 787 new
birds were fitted with a ring, the remaining birds being recorded as re-traps (see below). In addition to
these, 218 young birds were ringed in the nest, giving a grand total of 1005 new birds ringed compared
to 1288 last year.

4.3 RE-TRAPPING RINGED BIRDS

In addition to the 1005 new birds ringed, 293 recaptures of birds already wearing rings were made, five
of which were ringed at other sites (some details as yet unknown). One individual captured in 2019 was
originally ringed in 1981 and was not handled again until it was re-caught on 28 June 2019, 38 years and
one day after being ringed. The bird was originally ringed as an adult so its actual age when ringed is
unknown, other than it was probably more than four years old.

A selection of recovery details received from the BTO in 2019 can be found on page 218.

Fig. 4 Adult Manx Shearwater © Steven Stansfield

4.4 CHICK GROWTH RATES

Each week during the summer months, a visit was made to 36 of the most easily accessible productivity
shearwater nests to weigh and measure the chicks to monitor their growth rate. The first chicks were
measured on 24 June, ten days earlier than last year; the final measurement was taken on 19 September,
although the first had fledged by 28 August, eight days earlier than 2018. Several nests failed during the
season. The following results are based on the remaining 20 chicks from the original sample size of 36.
The maximum wing-chord to the nearest 1mm was taken, though this was not an easy measurement to
take when the chicks had first hatched, as the tip of the wing was fluffy. The birds were weighed using a
1 kg electronic balance.

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Wing LengthSHEARWATER PRODUCTIVITY, RINGING AND CHICK GROWTH

4.5 WING-LENGTHS24/06
26/06
Most chicks had a wing-length of between 20 and 30 mm, but some were as large as 36 mm or as small28/06
as 22 mm when first measured, with the mean first measurement being 26.65mm±s.d.3.28. The chicks30/06
continued to grow and fledged with an average wing-length of 229.81mm±s.d.7.05, 4.35%, shorter than02/07
an average adult bird from Bardsey at 240.27mm±s.d.4.75 (n2359).04/07
06/07
250 08/07
10/07
200 12/07
14/07
150 16/07
18/07
100 20/07
22/07
50 24/07
26/07
0 28/07
30/07
Date01/08
03/08
Fig. 5 Growth rate of chicks – wing-length by date 05/08
07/08
4.6 WEIGHTS 09/08
11/08
Most chicks had an initial weight of under 150g, with the mean initial weight being 126.67g ±s.d.71.35, 13/08
46.91g heavier when compared to last year’s mean initial weight of 79.76g ±s.d.40.25. The heaviest 15/08
weight recorded of all the chicks measured was 668g (3.47% lighter than last year’s heaviest bird at 17/08
692g); this was a chick that was first checked on 26 June and, on that date, weighed 77g. It reached its 19/08
peak weight on 29 July, slightly ahead of when the majority were at their heaviest, and fledged weighing 21/08
480g shortly after 26 August, having shed 188g, meaning it lost at least 28.14% of its mass. Most of the 23/08
chicks reached their peak weights in early to mid-August; the mean peak weight was 530.67g±s.d.119.18, 25/08
5.41% lighter when compared to last year’s mean peak weight of 561.06g ±s.d.51.08. The minimum peak 27/08
weight of any of the chicks was 461g (compared to 512g last year) from a chick whose fledging weight 29/08
was eventually 403g, amounting to a loss in weight of 58g, just 12.58% of its mass. All chicks this year were 31/08
generally of a slightly lighter weight than last year. 02/09
04/09
700 06/09
600
Weight 500
400
300
200
100

0

Date

Fig. 6 Growth rate of chicks – weight by date
30/06
04/07
08/07
12/07
16/07
20/07
24/07
28/07
01/08
05/08
09/08
13/08
17/08
21/08
25/08
29/08
02/09
06/09

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Fig. 7 Manx Shearwater from nest TP3 - photos taken weekly from July to September
© Sam Prettyman

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WeightSHEARWATER PRODUCTIVITY, RINGING AND CHICK GROWTH

Weight4.7 FLEDGING

Once the shearwaters reached a size where their wing-lengths were about 180-190mm (c75% of their
fledging wing-length) in mid-August, they began to slim down and lose weight prior to fledging. The
mean fledging wing-length was 231.55mm±s.d.6.87 (229.81mm±s.d.7.05 in 2018), with the smallest
fledgling having a wing-length of 216mm and the largest 246mm. The mean fledging weight was 475.37g
±s.d.41.67, 0.24% heavier than 2018’s 474.25g±s.d.38.01, the lightest being 403g and the heaviest being
560g, 11g heavier than the heaviest of 2018, which was 549g. The average weight loss, from peak weight
to fledging weight, was 78.11g±s.d.45.11, 10.02% less than 2018’s mean weight loss of 86.81g ±s.d.44.70,
with the largest weight loss being 188g (179g in 2018) from 668g to 480g (28.14% of the chick’s mass).

800
700
600
500
400
300
200
100

0
0 25 50 75 100 125 150 175 200 225 250 275
Wing Length

Fig. 8 Growth rate of chicks – weight against wing-length

560
540
520
500
480
460
440
420
400
380
360

210 212 214 216 218 220 222 224 226 228 230 232 234 236 238 240 242
Wing

Fig. 9 Fledging size of chicks – weight against wing-length

Adult Manx Shearwaters on Bardsey have an average wing-length of 240.27mm ±s.d.4.75 (n2359), with
a range of 223mm to 256mm. The mean fledging wing-length of the chicks in 2019 was 231.55mm, 3.77%
shorter than the average for adult birds and 1.74mm longer than the mean for birds that fledged in 2018.

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5 GROWTH RATES 2015 TO 2018

5.1 SAMPLE SIZE

Since 2015, an additional study of the productivity burrows has been carried out each summer to monitor
the growth rate of Manx Shearwater chicks. The nests were visited once a week from when the chicks
first hatched until they had fledged. On these visits the maximum wing-chord and weight was measured.
In addition, the chick was ringed once it had reached a suitable age. These data were compiled and
only birds that fledged were included in the results. The sample size varied from year to year depending
on the productivity of the burrows chosen; 28 were monitored in 2015, 26 in 2016, 21 in 2017, 16 in 2018
and 20 in 2019. The following results are based on the sum of these figures, with a resultant sample size of
111 chicks.

5.2 WING-LENGTH

When the first measurements were taken, chicks generally had a wing-length between 15 and 52mm,
with an average of 29.95mm±s.d.9.66; thereafter wing-length grew significantly for the next five weeks,
with a mean growth of 26.89mm±s.e.1.40 per week. Growth per week was variable through the season,
with wing-lengths reaching a maximum mean growth of 40.40mm per week five weeks in, however, some
grew as much as 75mm in a single week. In the final weeks wing growth generally began to fall before
chicks reached their fledging size, reaching their lowest growth per week in the final two weeks. Chicks
fledged with a mean wing-length of 231.55mm ±s.d.6.87 and with a wing-chord ranging from 216mm to
246mm.

45Growth (mm)
40
35 7-8 8-9
30
25 7-8 8-9
20 19.31 14.50
15 6.46 5.96
10
34 24
5 8 6
0

1-2 2-3 3-4 4-5 5-6 6-7
Week

Fig. 10 Average weekly growth of wing-length in 2019 (mm/week)

Table 3 Growth of wing-length (mm/week)

Week 1-2 2-3 3-4 4-5 5-6 6-7
Mean 19.00 27.95 28.90 34.45 40.40 30.60
8.79 10.62 9.96 16.90 15.37 13.14
s.d.±
Max 33 47 60 75 72 59
Min 7 9 10 17 18 15

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5.3 WEIGHTS

On the first visit, the chicks had a mean weight of 130.00g±s.d.71.46, but ranged from as low as 38g up
to 250g (this was a large chick that was not found in the first two weeks, making it a large chick on its first
visit). In the initial weeks, both adults returned to shore most nights to feed their chick, which rapidly
gained weight, with an average increase of 150.85g in the first two weeks. Over the first four weeks the
chicks increased on average by approximately 95g per week. However, by the fifth week adults returned
to the nest less frequently and chicks grew by a mean weight of just 25.45g±s.d.41.83 per week. Most
chicks reached their peak weight seven weeks after hatching at a maximum mean weight of
530.45g±s.d.63.74. One bird reached a peak weight of 668g whilst another significantly smaller bird
peaked at just 461g. Weight gain continued to fall throughout the last weeks of chick rearing until the
last two or three weeks, when birds began to slowly lose weight, having been abandoned by their
parents. One bird lost as much as 118g in a single week before fledging, which constituted 28.30% of its
fledging weight of 417g. Another bird lost just 24g and fledged at a weight of 456g, down from its peak
weight of 480g.

200Weight difference (gms)

150

100

50

0

-50

-100
1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9
Week

Fig. 11 Average weekly weight difference (±g/week)

Table 4 Weight difference (±g/week)

Week 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9
Mean 150.85 91.35 86.50 49.10 22.65 -12.30 -40.63 -30.83
58.90 38.78 46.74 35.34 44.04 51.75 44.04 12.46
s.d.± 146 195 126 107
Max 284 92 52 -13
Min 48 18 21 -2 -39 -118 -110 -45

5.4 FLEDGING

Having reached their peak weight, the shearwaters began to lose weight in the final three weeks before
fledging. The median fledging time, with 56 of 111 birds fledging, was after ten weeks. Some remained
in the burrow for as long as 11 weeks or fledged after just seven weeks. The mean fledging wing length
was 231.55mm±s.d.6.87, but one individual fledged with a wing as short as 216mm, 10.10% below that of
an average adult Manx Shearwater on Bardsey; another individual fledged with a wing of 246mm.

The mean fledging weight was 476.40g±s.d.40.82, with the smallest fledgling leaving the nest with a
weight of just 403g and the largest with a weight of 560g. In the final weeks, before fledging, chicks lost
a substantial amount of weight as they converted their fat reserves into muscle and feathers, the mean
weight-loss being 77.85g±sd.43.92. One bird lost 188g, 141.49% more than the average of 77.69g, having

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reached its peak weight of 668g in the fifth week. It dropped to 480g in week eight, having lost 28.15%
of its original weight, before fledging shortly thereafter.

6 DISCUSSION

Productivity increased this year, after 2018 experienced the worst productivity since monitoring began
in 1998. Despite this year’s upward movement, the overall productivity of Manx Shearwaters on Bardsey
has fallen by 0.18 chicks per pair or 23.08% since 1998. Over the years the productivity has ranged from
0.88 in 2003 to 0.60 in 2018. Whilst the productivity has fluctuated dramatically throughout the years, the
trend clearly shows a general decline. One of the reasons for the decline could be the increasing
population on Bardsey, with the 2014-2016 census producing 20,675 pairs, up from 15,859 only six years
earlier and the current census appears to indicate a further 20% increase, with a current estimate of
almost 25,000 pairs. This dramatic rise in population will result in more new and inexperienced pairs
breeding on the island, which could in turn lead to a lower productivity. The more favourable weather
experienced this year is likely to have played a part in the increase, but it will be the monitoring in the
coming years that will determine whether or not the population is growing.

Much like 2018 there was no clear pattern in terms of productivity in relation to habitat or location;
productivity simply varied across the island. The West Coast does show a low figure, but the sample size
was small. There is no immediate explanation for the increased productivity this year, with the exception
of the weather, which was mild all spring, especially when compared to the spring of 2018. The failures
mostly occurred at the egg stage; once the chicks hatched, all but one reached a fledgling stage. As
in 2018, some down was found at the entrance to two of the failed burrows, indicating that predation
played a part in the lack of success for some pairs.

7 SUMMARY

In total 122 burrows containing birds incubating eggs were found and subsequently marked. Productivity
was calculated from these burrows and a total of 94 chicks fledged. This gave a productivity of 0.77
chicks per pair, an increase of 28.33% on 2018 (0.60) and 12.90% higher than the ten-year mean
(0.68±s.e.0.02); this is 4.67% lower than the mean since 1998 (0.74±s.e.0.02) when monitoring productivity
began.

Ringing and re-trapping birds was again successful, with 1298 birds handled, and of these 787 new full-
grown birds were ringed. A further 293 birds were recaptured, including 24 that were re-ringed. Four
individuals over 30 years old were caught. One individual captured in 2019 was originally ringed in 1981
and was not handled again until it was re-caught on 28 June 2019, 38 years and one day after being
ringed. The bird was ringed as an adult so its actual age when ringed is unknown, other than it was
probably more than four years old. In addition to the 787 full grown birds a further 218 young birds were
ringed in their nests, giving a grand total of 1005 birds fitted with new rings during the year.

A sample of young was weighed and measured weekly during the nesting season, from hatching to
fledging. The mean fledging wing-length was 231.55mm, the smallest being 216mm and the largest
246mm. The mean fledging weight was 476.40g, 0.45% heavier than 2018’s 474.25g, the lightest being
403g and the heaviest being 560g. The mean weight loss was 77.85g, with a maximum loss of 188g.

8 ACKNOWLEDGEMENTS

Thanks are due to Natural Resources Wales for providing funding for the productivity monitoring to be
completed. Thanks also to Alex Starace, George Dunbar, Lewis Hooper and others who helped with
monitoring the burrows and measuring and ringing of the chicks.

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Fig. 12 Manx Shearwater chick removed from burrow for measuring © George Dunbar

9 REFERENCES

Else, R.J. 2009. Census of the breeding population and productivity monitoring of Manx Shearwaters
Puffinus puffinus on Bardsey in 2008. Rep. Bardsey Bird Fld. Obs. 52: 92-96.

Else, R.J. 2011. Manx Shearwater population census and productivity monitoring. Rep. Bardsey Bird Fld.
Obs. 54: 111-113.

Leaper, G. 2002. Census of the breeding population of Manx Shearwaters Puffinus puffinus on Bardsey
2001. Rep. Bardsey Bird Fld. Obs. 45: 93-99.

Perfect, E. 2018. Manx Shearwater Studies: productivity, ringing and chick growth. Rep. Bardsey Bird Fld.
Obs. 61:190-201.

Stansfield, S.D. 1998 - 2016. Reports to Natural Resources Wales. Unpublished.
Stansfield, S.D. & Carter, M. 2017. Manx Shearwater Studies: Manx Shearwater 2014-2016 population

census. Rep. Bardsey Bird Fld Obs. 60:152-158.
Walsh, P.M. et al. 1995. Seabird Monitoring Handbook for Britain and Ireland. JNCC/RSPB/ITE/Seabird

Group, Peterborough.

63: 164-177 177

Manx Shearwaters MANX SHEARWATER HOOPER
Population census 2019-2022 – year 1
© Steven Stansfield Lewis Hooper, Josie Hewitt and Steven Stansfield

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1 INTRODUCTION

The Manx Shearwater Puffinus puffinus is a pelagic seabird in the family Procellariidae, more commonly
known as the tube-noses (Brooke, M., 2013). It is the most northerly Atlantic breeding shearwater species,
and the UK holds more than 90% of the world’s population (Perrins et al. 2011). Bardsey is the fourth largest
Manx Shearwater colony in the world and the third largest colony in Wales, with only Skomer and
Skokholm holding larger populations (Perrins et al., 2012). The national and international importance of
Bardsey’s Manx Shearwater population is the reason for its listing as one of the avian ‘features’ of the
Bardsey Site of Special Scientific Interest (SSSI) and National Nature Reserve (NNR).

Owing to the island’s importance for Manx
Shearwaters, it is crucial that the
population is monitored on a regular basis.
This is done by way of a population census
which is carried out every six years. Over
the last two decades the Manx Shearwater
population on Bardsey has undergone a
significant increase, from less than 7,000
apparently occupied burrows (AOBs) in
1996 to over 21,000 in 2016 (Stansfield &
Carter, 2017). The last census took place
over the 2014-2016 breeding seasons
(Stansfield & Carter, 2017), however the
recent population increase means that the
current census will take four seasons to
complete, from 2019-2022.

2 AIMS

To comply with Natural Resource Wales

(NRW) requirements and the regulations for

monitoring features of the SSSI and NNR,

the Manx Shearwater population on

Bardsey Island is censused on a regular

basis using a consistent method. Doing so

enables comparable, long-term

monitoring of the population and

detection of any population changes. Fig. 1 2019 Manx Shearwater census area
(unshaded).
3 METHODS

In previous years, the island has been divided into three sections for the purpose of the census, with one
section surveyed per year. However, the expected population increase for the current census means it
is now necessary to further sub-divide the East Side section - 75% of the island’s population - in two. The
south-western section (unshaded area of Fig. 1) was surveyed in 2019, with the two halves of the East
Side due to be surveyed in 2020 and 2021 and the west side of the Mountain and North End to be
surveyed in 2022. The 2019 survey of the south-western region included the South End, the Narrows, the
West Coast, the Lowlands up to Carreg Bach, and a section bellow Tŷ Pellaf . The grid lines SH120 east
and SH220 north were used as the boundaries for the 2019 survey area.

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The methods used in the current census follow those used in previous years which are outlined in the
Seabird Monitoring Handbook (Walsh et al. 1995). However, following the low response rates obtained
on Bardsey in both 2008 and 2014 surveys, R. Brown and C. M. Perrins (pers. comm.) advised that using
dual-sex playback would improve response rates.

Results from Perkins et al. (2017) show that dual-sex calls
give higher response rates with lower variance than
male-only calls, since females frequently reply to
female calls but rarely to male calls. In simulated 3–5-
day calibration trials by Perkins et al. (2017), response
rates and correction factors were up to 50% more
precise with dual-sex calls. This suggests that females
are more likely to respond to female rather than male
calls, especially if they already have a partner and are
incubating an egg. As shown by Perkins et al. (2017)
and Perrins et al. (2012), male response rates remain
the same since they respond to the calls of other males
in defence of the burrow and their partner. The findings
from these studies show that dual-sex calls generate
increased response rates from birds in burrows, so this
minor change to the method was adopted for the
current census.

R. Brown and C. M. Perrins (pers. comm.) also advised Fig. 2 Easy Acc Mini Bluetooth speaker

that the low response rates in 2014 and 2018 could

have been compounded by the poor sound quality of the playback equipment used. To combat this

issue, an EasyAcc Mini Bluetooth Speaker – the same as is used on Skokholm (R. Brown pers. comm.) -

was purchased, and the MP3 dual-sex playback calls used on Bardsey from 2019 onwards are those used

by Perkins et al. (2017).

The 2019 census was conducted between 13 May and 14 June. This five-week period was chosen to
coincide with the peak incubation period as per Perkins et al. (2017). All Manx Shearwater burrows within
the census area were surveyed, and three separate counts were made simultaneously. All surveys were
conducted during the day, as fewer non-breeding or prospecting birds will be present in the burrows
(Walsh et al., 1995).

3.1 COUNT 1: THE TOTAL NUMBER OF BURROWS

Every burrow encountered was counted. Each burrow was checked to make sure the burrow is
complete (at least 30cm deep), and not just the start of a burrow, or a complex of burrows with multiple
entrances - see Fig. 3.

3.2 COUNT 2: APPARENTLY-OCCUPIED BURROWS (AOB)

Rabbits are no longer found on Bardsey and there are relatively few Puffins breeding on the island (c.150
pairs; Dykes & Stansfield, 2019), so any suitably sized burrow is most likely that of a Manx Shearwater. All
burrows are examined to determine whether or not they are ‘apparently occupied’; the majority of
burrows show clear signs of use (smooth, compacted soil entrance, parallel grooves made by the birds'
feet, fresh digging, droppings). However, not all burrows are this obvious, so this method can be rather
subjective. This is particularly true for burrows whose entrances lie between stones in walls or are almost
completely hidden by grass on the tops of walls and mounds; these burrows show little or no external
signs of use.

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Fig. 3 Apparently Occupied Burrows on Pen Cristin © Steven Stansfield

For burrows with little or no external signs of use, call playback helped to determine whether they were
occupied or not – any burrow where playback elicited a response, and which had a long tunnel of
suitable diameter was counted as an AOB. Burrows with dense vegetation growing in the entrance and
tunnel, or old eroded burrows were not counted. To complicate matters, some burrows have multiple
entrances, while in other cases entrances with signs of occupation can sometimes lead only to shallow
excavations just a few inches long or a large entrance might lead to two or more nesting chambers.

Burrow entrances were thoroughly checked for these features and if entrances were linked, only one
burrow was counted. Similarly, a response from two or more entrances was only counted once if it was
clearly from the same bird. In certain habitats - boulders and areas of dense European Gorse Ulex
europaeus - it was not possible to carry out a complete count of AOBs. Some burrow entrances are
distinguishable under boulders (soil worn smooth, droppings etc.) and under gorse bushes (tunnels in
vegetation under bushes, droppings), and these were counted, but many more may have been hidden.
In these areas a slight alteration to the survey methods was made c.f. 3.3 below.

3.3 COUNT 3: RESPONSE TO THE VOICE PLAYBACK

For the current census, a dual-sex call (male and female birds calling simultaneously) was used for the
voice playback survey. The recording was played into every burrow at a natural volume for 25 seconds
and was followed by a 25 second period of listening for any responses. If a bird called back during the
playback or listening time, then a response was noted. If a response was noted from two or more
neighbouring entrances, but was clearly from the same bird then it was counted only once.

Although most of the habitat in the 2019 census was clear stone walls, banks and grazed fields, some
areas contained dense vegetation including European Gorse, tall grass and bracken, which made it
difficult to count burrows and AOBs accurately. Some burrow entrances are still distinguishable in these

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Fig. 4 Newly excavated shallow burrow © Steven Stansfield

habitats, e.g. under boulders (soil worn smooth, droppings etc.) and under gorse bushes (tunnels in
vegetation under bushes, droppings). Those that could be distinguished were counted, but some are
likely to have been missed in areas of very dense vegetation. Due to the constraints posed by dense
vegetation, an adapted methodology was used in these areas; the dual-sex recording was played at
maximum volume into and around the tunnels or any areas showing signs of activity (droppings or
scrapings) and also at large areas of gorse for a period of 30 seconds, with a 60 second listening period.
Any responses were counted as an AOB.

3.4 SURVEY EFFORT

Traditionally, the island has been divided into three sections and surveyed over a three-year period.
However, due to the population increase over recent decades, the island has now been split into four
areas and will be surveyed over a four-year period (2019-2022). The 2019 census area was sub-divided
by walls and field boundaries; burrows were counted using click-tally counters and marked on maps in
the field. These were then transcribed onto a complete map of the whole census area at the end of the
census period.

The 2019 census was completed by a single member of Bird Observatory staff who was assisted by a
number of volunteers throughout the census period. Census fieldwork was always carried out during the
day in favourable weather conditions, and whilst the weather was conducive for most of the census
period, strong winds and rain did prevent fieldwork on a handful of days.

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4 RESULTS

A total of 3550 burrows was counted within the 2019 census area, of which 3078 (86.70%) were
considered to be apparently occupied; responses to voice playback were recorded from 1423 (46.23%)
of apparently-occupied burrows (AOBs).

Table 1: Results of the Bardsey 2019 Manx Shearwater population census.

South End Number of Apparently % of burrows Playback % of responses
North of the burrows occupied occupied responses from AOBs
(from AOBs)
Narrows 1128 burrows 84.13 36.99
Whole area 2422 949 87.90 351 50.35

total 3550 2129 86.70 1072

3078 1423 46.23

Table 2: Comparison of the 2008, 2014 and 2019 Manx Shearwater surveys on Bardsey.

South End 2019 2014 2008 % change
Number of burrows 1128 911 722 2014 – 2019 +23.82
949 836 632 2008 – 2019 +56.23
AOBs 351 105 224 2014 – 2019 +13.40%
Responses 2008 – 2019 +50.00%
2019 2014 2008 2014 – 2019 +234.29%
North of the Narrows 2422 1979 1742 2008 – 2019 +56.70%
Number of burrows 2129 1680 1546
AOBs 1072 247 499 % change
Playback responses 2014 – 2019 +22.39%
2019 2014 2008 2008 – 2019 +39.04%
Whole area total 3550 2890 2464 2014 – 2019 +26.73%
Number of burrows 3078 2516 2178 2008 – 2019 +37.71%
1423 352 723 2014 – 2019 +334.01%
AOBs 2008 – 2019 +114.83%
Playback responses
% change
2014 – 2019 +22.84%
2008 – 2019 +44.07%
2014 – 2019 +22.34%
2008 – 2019 +41.32%
2014 – 2019 +304.26%
2008 – 2019 +96.82%

5.1 DISCUSSION

5.1.1 POPULATION INCREASE

The results of this year’s survey show a marked increase in the total number of burrows, with the combined
number of burrows (AOBs and unoccupied) rising from 2464 in 2008 to 2890 in 2014 and then to 3550 in
2019 (Table 1). This is an increase of 44.07% over the 11-year period 2008-2019 and a 22.84% increase
from the previous census in 2014 (Table 2).

The number of AOBs showed a similar percentage increase to that of the total number of burrows, with
3078 AOBs counted in 2019. From 2008 to 2014, the number of AOBs increased from 2178 to 2516, a total
of 338 (15.52%). The number of AOBs rose further between 2014 and 2019, with an increase of 562

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(22.34%). Overall, the number of AOBs increased by 900 (+41.32%) between the 2008 and 2019 surveys
(Table 2).

These results show that the total number of burrows and those that are apparently-occupied are
increasing at a similar rate, perhaps suggesting that prospecting and excavating birds are returning to
take up residency and breed. It also suggests that the capacity for breeding Manx Shearwaters on
Bardsey has not yet been reached.

5.1.2 OCCUPANCY RATES

Burrow occupancy rate is broadly similar across the three survey periods (2008, 2014 and 2019), however
there has been a slight decrease in the percentage of occupied burrows. In the 2019 survey, 3078 of the
3550 (86.70%) were apparently occupied, compared to 87.06% in 2014 and 88.39% in 2008. This is a
decrease of -0.36% since 2014 and -1.69% since 2008 and could suggest that there are more non-
breeding immatures and excavating birds in recent years.

5.1.3 RESPONSE RATES

One of the most notable differences in this survey, compared to previous years, is the number of birds
that responded to the voice playback. In total, 1423 (46.23%) of the 3078 AOBs had responsive birds
inside. This is a large increase on the response rates in previous years, when responses were recorded
from 723 (33.20%) of 2178 AOBs in 2008 and 352 (13.99%) of 2516 AOBs in 2014. Due to time constraints,
the gender of each bird responding was not noted, so the true effect of female responses in this study is
unknown.

The response rates calculated on Bardsey in 2019 are 6% higher than those calculated on Skomer in 2011
(Perrins et al. 2012) and 17% greater than those on Rhùm in 2001 (Murray et al. 2003) where male-only
calls were used.

5.1.4 IMPROVED AUDIO QUALITY AND DUAL-SEX CALLS

It is well documented that male Manx Shearwaters have a response rate of 93-98% to calls of other males
during the breeding season and that females respond less than 1% of the time to the call of a male
(Perkins et al., 2017). As mentioned previously, dual-sex calls were used in the 2019 voice playback survey
on Bardsey as a direct result of a study by Perkins et al. (2017) which found that females respond at higher
rates to calls of another female compared to male-only calls. This decision seems to have worked well,
with a 96.82% increase in 2019 response rates (compared to 2008), however audio quality may also be
a contributing factor.

The same voice-playback methods were employed in the 2008-2010 and 2014-2016 surveys whereby
male-only calls were used. The only difference was the type of portable speaker used: two small
Dictaphones with built in speakers were used in 2008-2010, while two iLuv speakers and iPods were used
for the 2014-2016 surveys. A significant deterioration in the sound quality of the iLuv speakers over the
census period is believed to be the main reason for such poor response rates from birds in 2014-2016.

During 2015 and 2016 a test was carried out using the iLuv speakers and iPods to try and elicit responses
from incubating birds in 30 study burrows. Over a 14-day period, each burrow was surveyed four times
following the same voice playback methodology as used in the 2008 and 2014 censuses (25 seconds of
playback using male-only calls, followed by a 25 second listening period). Given that the process
occurred over the 14-day period, both males and females should have been incubating eggs in burrows
at different times and so a response rate of ~50% was to be expected. Consequently, it was surprising to
find that the response rates varied from c15-25% (pers obs./Stansfield and Leatherbee - unpublished

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Fig. 5 Surveyor listening for responding shearwaters © Steven Stansfield

data). It was concluded that the combination of poor-quality audio playback equipment and the use
of male-only calls was the reason for this low response rate.

As previously stated, the 2019 results saw a marked increase in responses, presumably due to the
methodology (dual-sex playback) along with the equipment used. The new EasyAcc Mini Bluetooth
Speaker and digital MP3 dual-sex recording of Manx Shearwaters produced a much better sound quality
at a greater volume; the improvement in sound quality alone could be expected to improve the
likelihood of a bird calling back to it. Studies conducted on Skomer (Perrins et al. 2012) suggest that the
call back rates obtained in 1998 and 2011, using very different audio tape qualities, produced similar
response rates, however we do not know how poor the 1998 equipment was compared to the Bardsey
equipment of 2008 and 2014. Booker et al. (2019) suggested that volume could also be a contributing
factor for poor response rates as they had issues with this among their surveyors on Lundy.

5.1.5 OTHER OBSERVATIONS AND CONSIDERATIONS

The results of the 2019 surveys suggest that Bardsey’s Manx Shearwater population is still expanding and
that the island’s carrying capacity for nesting Manx Shearwaters has not yet been reached. However, it
is important to remember that this is only the first year of a four-year survey across the island, so these
results do not necessarily represent the population dynamics of nesting Manx Shearwaters across the
remaining parts of the island. Despite this, the amount of new excavations observed across the whole
island does suggests that similar increases will be evident in the other areas too - see Fig. 4).

Due to the improved results with voice playback in the 2019 survey, a correction factor study will be
undertaken during the 2020 and 2021 breeding seasons on a sample of known occupancy burrows. A
correction factor will then be calculated and applied to the responses to voice playback in order to
generate a more accurate population estimate.

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6 SUMMARY

The results from the 2019 surveys suggest that the population of Manx Shearwaters on Bardsey is
increasing, and by a higher rate than a decade ago. A total of 3550 burrows were counted across the
2019 study area; 3078 (86.70%) were deemed to be apparently occupied, and voice playback elicited
responses from 1423 (46.23%) apparently-occupied burrows. Furthermore, the total number of burrows
and burrow occupancy rate are increasing at similar rates (approx. +22% since 2014).

7 ACKNOWLEDGEMENTS

Thanks are due to George Dunbar, Vic Bevan, Tom Williams, and Ed Betteridge for their assistance with
the census.

8 REFERENCES

Booker, H., Price, D., Slader, P. Frayling, T., Williams, T., and Bolton, M. 2019 Seabird Recovery on Lundy
British Birds 112: 217-230

Brooke, M., 2013. The Manx Shearwater. T&AD Poyser, London.
Murray, S., Shewry, M.C., Mudge, G.P. and Spray, S., 2003. A survey of Manx Shearwaters Puffinus puffinus

on Rum, Inner Hebrides in 2001. Atlantic seabirds, 5: 89-100.
Perkins, A.J., Douse, A., Morgan, G., Cooper, A. and Bolton, M., 2017. Using dual-sex

calls improves the playback census method for a nocturnal burrow-nesting seabird, the Manx
Shearwater Puffinus puffinus. Bird Study, 64:146-158.
Perrins, C.M., Wood, M.J., Garroway, C.J., Boyle, D., Oakes, N., Revera, R., Collins, P. and Taylor, C., 2012.
A whole-island census of the Manx Shearwaters Puffinus puffinus breeding on Skomer Island in
2011. Seabird, 25:1-13.
Stansfield, S., and Carter, M., 2017. Manx Shearwater studies: Manx Shearwater 2014- 2016 population
Census. 2016 Bardsey’s Wildlife. Rep. Bardsey Bird Fld. Obs. 60: 152-158.
Walsh, P.M., Halley, D.J., Harris, M.P., Del Nevo, A., Sim, I.M.W. and Tasker, M.L., 1995. Seabird monitoring
handbook for Britain and Ireland: a compilation of methods for survey and monitoring of
breeding seabirds. JNCC/RSPB/ITE/Seabird Group.

Fig. 6 Manx Shearwaters ashore at night © Steven Stansfield 187

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CHOUGH

A summary of the 2019 breeding season
Lewis Hooper

Chough REP. BARDSEY BIRD FLD OBS

© Steven Stansfield

188

CHOUGH

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1 INTRODUCTION

Chough Pyrrhocorax pyrrhocorax breed across much of central Asia, southern and eastern Europe and
Morocco, with small outlying populations in the Canary Islands, Ethiopia and Sudan. Historically, Chough
bred around much of the UK’s coastline and some parts of inland Scotland. However, thanks to changes
in livestock management and indiscriminate persecution, the UK population underwent a steep decline
and range contraction during the late 1800s, with the only remaining birds breeding in their current
strongholds (Bullock et al., 1983). By the early 20th century, Chough populations in England and Scotland
continued their downward trajectory, while those in Wales and on the Isle of Man showed some signs of
recovery.

The most recent national census of Chough in the UK and the Isle of Man took place in 2014, and
estimated the population to be 433 breeding pairs, with the majority found in Wales (55%) and the Isle of
Man (31%; Hayhow et al., 2018). Overall, numbers of Chough in the UK and the Isle of Man have remained
stable between the two most recent national surveys in 2002 and 2014, but these population trends
exhibit some geographical variation with increases in the Isle of Man, Cornwall and south Wales
contrasted by declines in Scotland and parts of north and mid-Wales.

Chough are heavily protected in the UK and Europe since they are listed under Schedule 1 of the Wildlife
and Countryside Act 1981 as well as Annex 1 of the EC Conservation of Wild Birds Directive 1979. The
Bardsey population is further protected as they are a feature of the Ynys Enlli Glannau Aberdaron Special
Protection Area (SPA) and Bardsey Island Site of Special Scientific Interest (SSSI), in addition to being listed
in Gwynedd’s Local Biodiversity Action Plan.

STATUS AND LEGAL PROTECTION SUMMARY

• Listed as a protected species under Annex I of the EC Conservation of Wild Birds Directive 1979
(2009/147/EC)

• Protected under Schedule 1 of the Wildlife and Countryside Act 1981
• A designated feature of the Ynys Enlli SSSI
• Feature of the Ynys Enlli and Glannau Aberdaron SPA

• Listed in the Gwynedd Biodiversity Action Plan

Number of pairs 10
9
8 1958 1963 1968 1973 1978 1983 1988 1993 1998 2003 2008 2013 2018
7
6
5
4
3
2
1
0
1953

Year Mean 4.52 ±s.d.2.24
Number of pairs

Fig. 1 Bardsey Chough breeding population 1953-2019

The Chough population on Bardsey has increased substantially since the Observatory began monitoring
them in 1953. Prior to the start of this century, an average of 4 pairs nested each year, but since 2001 this

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CHOUGH

has increased to ~7 pairs annually. Seven pairs bred on Bardsey in 2019, which is less than in each of the
two preceding years. The long-term mean number of pairs for 1953-2019 is 4.52 (±s.d.2.24), with the
current breeding population standing 54.76% above this long-term average.

Table 1. Breeding data 1953-2019. The average number of breeding pairs, average number
of chicks fledged and average productivity 1953-2019.

1953-2019 Average number Average number Average productivity:
1953-1970 of breeding pairs (±s.d.) of chicks fledged (±s.d.) chicks/pair (±s.e.)
1973-2000 2.66 ±0.13
2001-2010 4.51 ±2.25 11.34 ±6.05 3.27 ±0.29
2010-2019 2.11 ±0.96 6.24 ±3.03 2.48 ±0.18
4.29 ±1.24 2.79 ±0.28
6.00 ±0.67 10.89 ±5.03 1.88 ±0.16
8.10 ±0.99 16.90 ±6.30
15.40 ±4.81

Productivity4.50 30
19984.00 25
19993.50 20
20003.00 15
20012.50 10
20022.00 5
20031.50 0
20041.00
20050.50
20060.00
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019

Number of pairs and young

Year

Pairs Productivity young

Fig. 2 Number of Chough pairs, their chicks and productivity rates 1998 – 2019.

2 AIMS

Chough are a designated feature of the Bardsey Island SSSI and NNR. As such, the Welsh government
have a statutory obligation to monitor the population. This is done through the countryside agency,
Natural Resources Wales (NRW), who in turn contract BBFO to undertake the work through Section 16 of
the Wildlife and Countryside Act Management Agreement.

3 METHODS

Choughs were observed during the early part of the nesting season and their nests located. Each pair
was observed for up to two hours per visit to confirm breeding and other activity. Chicks in all of the
accessible nests were fitted with a single metal and three plastic colour rings in a unique combination
that allows them to be individually identified in the field post-fledging. All nest visits and ringing were
conducted under the appropriate licences from NRW and the British Trust for Ornithology (BTO). Once
the chicks fledged, they were monitored until they left the island.

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4 RESULTS

Fourteen traditional nest sites were checked and monitored during the 2019 breeding season, with 7
pairs attempting to breed. This is one less than in 2018, and is due to one of the regular pairs being
incomplete. Four pairs were confirmed to successfully fledge young, with a total of 11 chicks fledging (1
x 4 chicks, 2 x 3 chicks and 1 x 1 chick). A further three juveniles of unknown origin were subsequently
seen around island; it is possible that they were from a nest at the Zawn, where a pair were apparently
feeding chicks, but were not confirmed to fledge any young.

The number of fledged chicks in 2019 is lower than in previous years with a total of 22 fledged in 2017
and 15 in 2018 (see Table 1) and is 28.57% below the ten-year mean (15.40 ±s.d.4.81). Chough
productivity in 2019 was also lower than in recent years - 1.57 in 2019, compared to 2.44 in 2017 and 1.88
in 2018 (Table 1) and the 2019 figure is 16.57% lower than the ten-year mean (1.88 ±s.e.0.16).

Table 2. Breeding data 2010-2019

Year 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019
№ of pairs 6 8 9 9 8 9 8 9 8 7
№ of young 8 14 15 13 24 18 14 22 15 11
Productivity
1.33 1.75 1.67 1.44 3.00 2.00 1.75 2.44 1.88 1.57

Fig. 3. Female Chough from Ogof Trwyn-yr-hwch Bach © Steven Stansfield

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